fix: drop thinking deltas from background agent loops

Skip thinking-only deltas when accumulating background, scheduled-task, and teacher captured reply text.

src/task_scheduler.py

@@ -1649,6 +1649,8 @@ class TaskScheduler:
                     data = json.loads(event_str[6:])
                     # Capture text from all event types, not just delta
                     if "delta" in data:
+                        if data.get("thinking"):
+                            continue
                         full_text += data["delta"]
                     elif data.get("type") == "tool_output":
                         # Tool results — capture summary so we have SOMETHING even